The Universal Serial Bus (USB) has evolved from a data interface capable of supplying limited power to a primary provider of power with a data interface. Today many devices charge or get their power from USB ports contained in laptops, cars, aircraft or even wall sockets. USB has become a ubiquitous power socket for many small devices such as cell phones, MP3 players and other hand-held devices. The charger path of USB systems currently includes a buck regulator inside the charger and then again another series of switching regulators and/or buck regulators in a device being charged, to provide constant current and voltage for charging a battery, for example, a lithium battery. Each one of these regulators add inefficiencies to the charging path. The power lost from switching (e.g. stepping up or down to a charging current and/or voltage) becomes heat and hence a power management integrated circuit (PMIC) must be larger to compensate for heat dissipation. The combination of increased footprint and excess heat reduce overall performance.
While designs using two buck regulators (e.g. one in the charger and one in the device) can offer efficiencies of around 75%, they do not deliver optimal amounts of power to the battery to charge at maximal rates. A common strategy to combat this is to have VBUS output (of a USB port) at a higher voltage, at the same current output, which allows for higher power delivery to the system. However, this strategy creates a few challenges. For one, the system needs to have two buck regulators: a first one to make the inputs to system voltage regulators (e.g. LDOs (low dropout output regulators) and the like); and a second one to create a constant current constant voltage source for the battery. This also increased cost of the power path. Secondly, having to step down from an even higher voltage exasperates the heat dissipation problems.